It is believed that most, if not all, adsorbent medias conventionally used in arsenic removal service are adversely affected by high pH. While this can be addressed on a large scale in central water treatment systems by typical pH adjustment methods, such as the addition of a solution of acid to the feed liquid, a good method for adjusting pH in conjunction with a water treatment device for a household does not exist.
Adding chemicals to liquids to adjust pH is problematic. It is expensive and may raise regulatory concerns in connection with drinking water. In general, for pH lowering, sulfuric acid, hydrochloric acid, or carbon dioxide are used. For pH raising, solutions of sodium hydroxide, sodium carbonate, or a slurry of magnesium hydroxide are typically used. Inherent in any chemical addition system is a reliability issue. If there is a mechanical or electrical failure in the dosing system, or if the storage tank containing the acid is not periodically re-filled, the pH will revert to the feed water pH. For arsenic removal, this means the consumer could ingest high arsenic water. For some adsorption medias (e.g., iron based adsorptive media), failure of the pH adjustment system may result in release of adsorbed arsenic into the product water.
The water treatment industry would benefit from a system that is more reliable in that it avoids using electromechanical dosing devices for releasing chemical solutions to adjust pH and avoids using storage tanks of solution that need refilling.